The ATI Radeon is an interesting card to own, especially when compared to what nVidia offers. A few months ago ATI re-designed their nomenclature to a simple number-based system. The higher the number, the more powerful the card.
In general the Radeon is known for having somewhat superior image quality relative to it's GeForce competition, but at the expense of pure performance. ATI also offers an 'All-in-Wonder' series of Radeon which contains a wide range of multimedia features, including TV Tuner and even a remote in the latest version.
I won't go into the exact specifications of each card (you can find relative comparisons, as well as benchmarks, at Anandtech, Sharky Extreme, or HardOCP), but suffice to say their latest offering, the Radeon 8500, is generally considered a very nice card. Although it lags framerate-wise compared to the GeForce 4, the pricing is significantly less.
Special mention should be payed to the Radeon 8500DV AIW, which is possibly one of the most feature-rich cards on the market today. Although the memory and engine clock speeds are slightly slower than that of the non-AIW 8500, it offers a highly acclaimed TV Tuner as well as a radio remote for the couch potatoe in all of us.
One should always do proper research before upgrading any of their components, especially something as essential as the video card. The websites named above are an especially good resource for those looking to get a performance boost, or change any component on their PC.